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Technical ENSO Update

16 December 2010


>	Current conditions
>	Expected conditions

Current Conditions

As of mid-December 2010, SSTs continue to indicate moderate to strong La Niña conditions in the central and eastern equatorial Pacific. For November the SST anomaly in the NINO3.4 region was -1.45 C, indicative of moderate to strong La Niña conditions, and for the September-November season the anomaly was -1.53 C. Currently the IRI's definition of El Niño conditions rests on an index of SST anomalies, averaged over the NINO3.4 region (5S-5N; 170W-120W), exceeding the warmest 25%-ile of the historical distribution, and similarly for La Niña relative to the 25%-ile coldest conditions in the historical distribution. The NINO3.4 anomaly necessary to qualify as La Niña or El Niño conditions for the Dec-Jan-Feb and the Jan-Feb-Mar seasons are approximately (-0.65C, 0.65) and (-0.55, 0.50), respectively.

Expected Conditions

The most recent weekly SST anomaly in the NINO3.4 region is -1.5 C indicating moderate to strong La Niña conditions in the tropical Pacific; this is approximately the same as the -1.45 C level observed in November. What is the outlook for the ENSO status going forward? December is a time of the year during which the observed ENSO state usually indicates the ENSO conditions to be expected into the first one to three months of the following year, and, if there is an ENSO event in progress (as is currently the case) that event is typically peaking in strength and perhaps even starting to decrease slightly. One might ask whether the current La Niña condition is therefore guaranteed to persist over the coming several months, and whether it would be expected to begin weakening in the coming one or two months. A clue to the answers lies in the moderate strength and extent of negative subsurface sea temperature anomalies at depth in the central and eastern Pacific, whose upper portion has encompassed the surface waters since approximately mid-June. The subsurface negative heat content anomaly has slightly decreased from one to two months ago, but is still substantial. Sea temperature anomalies below the surface often portend SST anomalies to be expected in the following few months, as they are subject to the climatological upwelling in the eastern part of the basin. Current low-level wind anomalies indicate much enhanced trades, especially in the western and west-central portion of the tropical Pacific, and very strongly positive traditional and equatorial SOI indices. These atmospheric features imply above-average upwelling activity in the eastern tropical Pacific, at least for the near term, and suggest that the negative subsurface sea temperature anomalies will likely continue to find their way to the surface during the coming few months. The continued surfacing of below-average subsurface waters implies a likelihood for continuation of the strength of the currently moderate to strong La Niña conditions, although additional cooling appears unlikely at this point as we are near or even just past the usual time of peak strength. In view of the steady strength of the negative anomalies of the east-central tropical Pacific SSTs, we may have already seen the peak strength in October, but there could just as likely be a continuation of the currently moderate to strong SST anomaly level as there could be a slow decrease in their strength during the coming month. Above-average subsurface waters exist in the western tropical Pacific, and although they have been edging eastward during the last few months they do not appear poised to displace the large volume of below-average water to their east for at least a two or more months.

Presently, the models and observations taken together indicate probabilities of approximately 98% for maintaining La Niña conditions, near 2% for returning to ENSO-neutral conditions, and nearly 0% for developing El Niño conditions during the Dec-Feb 2010/11 season in progress. Going forward, probabilities for La Niña continue at 94% or greater through Jan-Mar 2011. In northern spring 2011 the probabilities for La Niña begin to weaken at a faster rate, declining to about 64% by Mar-May and to 43% for Apr-Jun.

The above assessment was made in part on the basis of an examination of the current predictions of ENSO prediction models as well as the observed conditions. For purposes of this discussion, El Niño SST conditions are defined as SSTs in the NINO3.4 region being in the warmest 25% of their climatological distribution for the 3-month period in question over the 1950-present timeframe. The corresponding cutoff in terms of degrees C of SST anomaly varies seasonally, being close to 0.40 degrees C in boreal late-spring to early-summer season and as high as 0.75 degrees C in late boreal autumn. La Niña conditions are defined as NINO3.4 region SSTs being in the coolest 25% of the climatological distribution. Neutral conditions occupy the remaining 50% of the distribution. These definitions were developed such that the most commonly accepted El Niño and La Niña episodes are reproduced.

The models show unanimous agreement regarding the continuation of La Niña conditions into the earliest months of 2011, but vary somewhat in their predictions of the rate of decreasing strength starting from Feb-Apr season. Most statistical and dynamical models call for at least moderate La Niña strength (stronger than -1C) through the Jan-Mar season, and all models except for one (of 22) indicate a waning of strength during Jan-Mar relative to the initial Dec-Feb season. For the Dec-Feb, Jan-Mar and Feb-Apr seasons, 100% of the models are predicting La Niña conditions, while none predicts ENSO-neutral conditions. From Mar-May onward, some models begin indicating a return to neutral ENSO conditions, but not to the point of being a majority until late northern spring and early summer. By Jun-Aug, a few models call for weak to moderate El Niño conditions. At lead times of 4 or more months into the future, statistical and dynamical models that incorporate information about the ocean's observed subsurface thermal structure generally exhibit higher predictive skill than those that do not. Among models that do use subsurface temperature information, 3 of 12 (25%) predict ENSO-neutral SSTs for the May-Jul seasons, 7 of 12 (58%) predict La Niña conditions, and 2 of 12 (17%) predict El Niño conditions. (Note 1). (Note that La Niña conditions for May-Jul require a NINO3.4 SST anomaly of -0.50 or stronger, and El Niño conditions require 0.45 or stronger.) Caution is advised in interpreting the distribution of model predictions as the actual probabilities. At longer leads, the skill of the models degrades, and skill uncertainty must be convolved with the uncertainties from initial conditions and differing model physics, leading to more climatological probabilities in the long-lead ENSO Outlook than might be suggested by the suite of models. Furthermore, the expected skill of one model versus another has not been established using uniform validation procedures, which may cause a difference in the true probability distribution from that taken verbatim from the raw model predictions.

An alternative way to assess the probabilities of the three possible ENSO conditions is to use the mean of the predictions of all models, and to construct a standard error function centered on that mean. The standard error would be Gaussian in shape, and would have its width determined by an estimate of overall expected model skill for the season of the year and the lead time. Higher skill would result in a relatively narrower error distribution, while low skill would result in an error distribution with width approaching that of the historical observed distribution. This method shows probabilities for La Niña at near 100% for Dec-Feb and Jan-Mar, remaining at 96% or more through Mar-May 2011. (Note that the threshold for La Niña weakens from approximately -0.65C to -0.40C between Dec-Feb and Mar-May, due to the seasonality of the interannual variance). The forecasters believe, however, that the Mar-May model-based probability may be higher than it should be, due to a common model bias of persisting ENSO episodes for too long a duration at the end of their typical seasonal cycle. Model probabilities for La Niña are 51% for May-Jul, and decrease to the climatological probability of 25% for Aug-Oct. The same cautions mentioned above for the distribution of model predictions apply to this alternative method of inferring probabilities, due to differing model biases and skills. In particular, this approach considers only the mean of the predictions, and not the range across the models, nor the ensemble range within individual models.

The IRI's probabilistic ENSO prediction takes into account the indications of this set of models, the outcome of the standard error approach described above, and additional factors such as the very latest observations that may have developed after the initialization times of some of the models. It indicates a 98% probability for La Niña conditions in the Dec-Feb season in progress, and only slightly less (94%) for Jan-Mar 2011. Probabilities for neutral ENSO conditions rise from 2% in Dec-Feb to 13% in Feb-Apr 2011, and to 45% by Apr-Jun. The probabilities for a return to El Niño conditions remain at very low levels from the present through early 2011, not exceeding 10% until Apr-Jun and reaching 23% by Jun-Aug.